Referring to FIG. 1, a communication system of this type will be described. FIG. 1 is a schematic block diagram showing the conceptual system configuration of the communication system of this type.
The illustrated communication system comprises first through fourth base stations 3-1, 3-2, 3-3, and 3-4, and these first through fourth base stations 3-1 to 3-4 have, as subordinates of them, first through fourth cells A1, A2, A3, and A4 serving as radio service areas, respectively.
In the example being illustrated, the first and the second base stations 3-1 and 3-2 are managed and controlled by a first radio network controller (RNC) 2-1, and the third and the fourth base stations 3-3 and 3-4 are managed and controlled by a second radio network controller 2-2. These first and second radio network controllers 2-1 and 2-2 are generally referred to as radio base station control stations, and hereinafter referred to as base station control stations in the present specification.
These first and second base station control stations 2-1 and 2-2 are connected to a core network 1 which is put into a higher position, and this core network 1 is illustrated on the assumption that it even includes a communication network. The core network 1 performs processing such as line connecting and switching processing between the communication network and a user terminal 4 serving as a mobile station within the first through the fourth cells A1 to A4. In the example being illustrated, the user terminal 4 is positioned in the second cell A2. This core network 1 essentially has a function as a switching center, and is therefore hereinafter referred to as the switching center in the present specification.
FIG. 2 shows, in sequence, an example of operation in a case where a user wishes to enjoy some sort of a communication service by means of the user terminal 4, in the communication system shown in FIG. 1.
In this event, first, the user terminal 4 makes an outgoing call that includes information indicative of the type of the service in question. This outgoing call will be sent to the second base station 3-2, the first base station control station 2-1, and the switching center 1. Responsive to this outgoing call, the switching center 1 sends an RAB (Radio Access Bearer) assignment request message to the first base station control station 2-1. In this event, this RAB assignment request message contains requested bit rate information in accordance with the communication service requested by the user terminal.
Responsive to the RAB assignment request message containing the requested bit rate information, the first base station control station 2-1 determines the state of use of resources (lines) that it manages. This state of use is also the state of use of the recourses within the second cell A2 serving as the service area for the second base station 3-2. If the resource that satisfies this requested bit rate can be ensured, the first base station control station 2-1 determines that the communication service at this requested bit rate is possible. The first base station control station 2-1 then conveys this fact as a RAB assignment response message to the switching center 1. Responsive to this response massage, the switching center 1 regards the call from the user terminal 4 in question as successful, and starts the communication service at the requested bit rate in question.
On the other hand, the first base station control station 2-1 always monitors the state of use of the resources that it manages. When determining that it is impossible to assign a line that can satisfy the requested bit rate due to lack of resources, the first base station control station 2-1 conveys, as the RAB assignment response message to the switching center 1, the intention that it will substitute a bit rate lower than the requested bit rate. Responsive to this response massage, the switching center 1 performs the operation in one of the two cases described below. In a first case, the switching center 1 regards the call from the user terminal 4 in question as successful, and starts the communication service at the bit rate lower than the requested bit rate. In a second case, the switching center 1 determines that the bit rate is not the bit rate suitable for the communication service requested by the user terminal, and transmits a RAB release request message to the first base station control station 2-1. Then, in this event, the RAB release response message is sent from the base station control station, and the switching center regards the outgoing call as unacceptable.
It should be noted that the communication services that the user would enjoy include such services as an image data service including still images and moving images, and image voice data service in which voice is added to images.
It will be assumed that the communication service that the user intends to enjoy is such that requires a high bit rate as a service of the image data similar to real-time video data. In this event, when communication is carried out at a bit rate lower than that, the user terminal can not receive a necessary amount of data in a short time interval due to the lowered transmission rate. As a result, the images displayed on the actual user terminal would be unnatural.
Therefore, when the base station control station, of its own accord, decides a lower bit rate other than the desired bit rate designated from the switching center 1 to the base station control station as described above, there is a problem that smooth data transmission can not be achieved because the bit rate resource that the user satisfies can not be ensured.
In addition, it will be assumed that the switching center determines that it is impossible to provide the service at the lower bit rate decided by the base station control station of its own accord. In this event, the switching center 1 needs to transmit a release request message, as shown in FIG. 2, in order to release the resource held by the base station control station, it is therefore disadvantageous in that unnecessary signal processing is increased.
It is therefore an object of the present invention to provide a communication system, a method thereof, a switching center thereof and base station control stations thereof that enable smooth data transmission although a communication rate corresponding to a communication service desired by a user can not be ensured due to lack of resources.
It is a specific object of the present invention to provide a communication system, a method thereof, a switching center thereof and base station control stations thereof that enable smooth data transmission, not by compulsorily setting to a lower bit rate, but by negotiating the bit rate between the switching center and the base station control station only when it is possible to provide a service at a bit rate lower than the desired bit rate, in the case where the base station control station can not ensure the desired bit rate due to lack of resources.